Recently, with an increase in the use of the Internet, the potential for digital subscriber loops (DSL) such as Asymmetric Digital Subscriber Loop (ADSL), High Speed DSL (HDSL) and Very High Speed DSL (VDSL) to provide benefits for communications has become apparent. Such a subscriber loop allows for increased transfer rates of downloaded data and uploaded data for a remote user. For example, a loop with 24 AWG twisted pair cable of about 18000 feet may be capable of transfer rates of 6 Mb/s downstream and 1 Mb/s upstream.
The data rates of an ADSL system are generally superior to those of a conventional telephone subscriber loop. Such a conventional subscriber loop, for example, may be capable of transporting 33.6 kb/s in the "voice band." The voice band is typically characterized as the frequency range 300-4000 Hz. The conventional subscriber loop may be limited by the use of hybrids at local exchanges and by line interface cards that allow the multiplexing of multiple voice bands on T1 carriers on the inter-exchange and inter-office networks of the telephone carriers. Line bandwidth may also be limited by the use of loading coils (typically found on loops of greater than 18000 feet in length) that serve to balance the loop periodically.
In order to achieve the high data rates of HDSL, ADSL and VHDSL, these digital subscriber loops typically bypass the inductive elements and load balancing components of the conventional telephone network. Such a bypass arrangement is illustrated in FIG. 1 which describes a conventional digitally switched telephone network. As seen in FIG. 1, a user at a remote premise 10 utilizes a telephone device 12 to communicate with a local exchange digital loop interface 14. The signal is provided to a hybrid circuit 16 which provides the signal to an analog to digital converter 18 which converts the analog signal to a digital signal. The digital signal is to multiplexer circuit 20 which packetizes and multiplexes downstream or de-multiplexed upstream. The upstream signal is then provided to the central switching office. In the downstream direction, the signal is received from the central switching office and operations proceed back through the multiplexer circuit 20, analog to digital interface 18 and the hybrid circuit 16.
As described above, ADSL allows for the asymmetric data rates in the downstream (network to subscriber) and upstream (subscriber to network) directions. ADSL also allows for concurrent transmission of conventional analogue telephone signals on the same circuit. ADSL uses the frequencies above the voice band (i.e. above 4000 Hz) to transfer high speed data between the network and the subscriber. ADSL may utilize a discrete multi-tone scheme, as described by the specification ANSI T1.413, for the transfer of data. The discrete multi-tone system of ANSI T1.413 also specifies two forms of encoding. A mandatory Reed Solomon code and an optional TCM code. These forms of encoding provide robustness in a noisy interference limited environment. In addition, data is interleaved over a long period of time, thereby protecting the transmission from localized disturbances such as impulse noise.
In a discrete multi-tone system, data is encoded into sub-carriers within the range of frequencies allocated for use in the discrete multi-tone system. The amount of data modulating a particular sub-carrier is determined during a training process where the impact of the characteristics of the ADSL connection are evaluated to determine the amount of data that each frequency may effectively support. This determination typically results in frequencies with a high signal to noise ratio being associated with higher rates of encoded data and frequencies with a low signal to noise ratio being associated with lower rates. Thus, ADSL typically utilizes the training process to determine the channel response and then the channel rates and coding that will maximize throughput for that channel response. Standards that describe conventional ADSL modems include ANSI T1.413 which describes Discrete Multi-tone Modulation (DMT).
Because ADSL typically allows for concurrent voice and data communications, ADSL modems typically include a splitter which separates processing of the voice band signal from the processing of the digital data signal. This separation typically must be capable of handling transients that occur on the phone line when calls are initiated and terminated to avoid such transients impacting the digital data signal. However, this ability to handle transients may adversely affect the throughput of the data signal. The splitter effectively isolates the ADSL signal and the voiceband signal from each other.
The splitter may also serve to protect the voice band signals from the data signals of the higher ADSL frequencies, which are typically higher power signals. Furthermore, the cost of installation of the splitter at a customer's premises may limit the widespread acceptance of ADSL. One example of a splitter assembly is illustrated in PCT Application No. WO 97/20396 to Russell et al. entitled POTS SPLITTER ASSEMBLY WITH IMPROVED TRANSHYBRID LOSS FOR DIGITAL SUBSCRIBER LOOP TRANSMISSION.
A conventional ADSL modem with a splitter is illustrated in FIG. 2. As seen in FIG. 2, the Internet 30 may provide a signal to a network bridge 32 which provided a signal to a xDSL modem 34. As used herein "xDSL" is a generic term denoting the entire class of DSL signals. Thus, the terms "DSL" and "xDSL" are use interchangeably. The xDSL modem then provides a signal to the splitter/combiner 36 which also receives a signal from a line termination of a POTS/ISDN line 44. The first splitter/combiner 36 then sends the combined signal to a second splitter/combiner 38 (typically at a remote location). The second splitter/combiner 38 receives the combined signal and may separate the POTS/ISDN signals from the DSL signals and provide the DSL signals to a xDSL modem 40. The xDSL modem 40 may then convert the DSL signals to data and provide the data to a bridge 42 connected to a second network, such as an Ethernet network. The POTS/ISDN signal from splitter 38 may be separately processed as a voice signal. Similarly, signals generated by the second xDSL modem 40 or POTS/ISDN line could be combined in the second splitter/combiner 38 and then separated in the first splitter/combiner 36 for communication in the reverse direction.
One solution which has been proposed to address the need for a splitter in an ADSL system is an ADSL modem which reduces the transmission power by 6-9 db in the ADSL frequencies to reduce spill over from the ADSL band to the voice band. Furthermore, the start tone of the upstream ADSL frequencies may be increased in frequency to thereby leave a larger buffer of inactive frequencies between the ADSL band and the voice band. A further proposed solution is to increase the coding on the ADSL data to increase the error correction of the data to compensate for errors caused by reducing the transmission power or by transients from the initiation or termination of calls. However, the consequence of these attempts to eliminate the splitter is that the data throughput of the ADSL modem will typically be decreased for all communication, including when the telephone instrument is not in use. For example, as a result of lower transmit power, the effective data rate for downstream communications may be reduced from 6 Mb/s to from 500 to 1000 kb/s.
A system using a modem which does not require splitters is illustrated in FIG. 3. As seen in FIG. 3, the DSL modems of FIG. 2 have been replaced with low power DSL modems 48 and 50 described above and the splitter/combiner 38 at the customer premises has been eliminated. Alternatively, the splitter/combiner may be eliminated at both the central office and the customer premises. In the system illustrated in FIG. 3, the ADSL and voice signals may both be provided to the line termination 44 or the telephone 46 as well as to the DSL modems 48 and 50. While the system of FIG. 3 may not use a splitter/combiner at both locations, as discussed above, data throughput will typically be reduced in comparison to the system of FIG. 2.
A system for the concurrent transmission of voice and data have is described in U.S. Pat. No. 4,757,495 to Decker et al. entitled SPEECH AND DATA MULTIPLEXOR OPTIMIZED FOR USE OVER IMPAIRED AND BANDWIDTH RESTRICTED ANALOG CHANNELS. Decker et al. describes a system which uses an ensemble modem which operates using carrier frequencies in the voice band and varies the amount of bandwidth in the voice band utilized for data based on the presence or absence of a voice call. When the modem changes the allocation of the voice band between voice and data communications the modem retrains to compute a new modulation scheme. The allocation between voice and data sub-bands may be based on user preferences of the desired speech quality.
U.S. Pat. No. 5,475,691 to Chapman et al. entitled VOICE ACTIVATED DTAT RATE CHANGE IN SIMULTANEOUS VOICE AND DATA TRANSMISSION describes a simultaneous voice and data modem which performs voice activated data rate changes. Thus, when the modem detects a local telephone call, the modem selects a signal space and a lower symbol density which provides for a higher quality voice transmission.
In light of the above discussion, a need exists for improvements in DSL modems which do not utilize a splitter so as to take better advantage of the benefits of separate frequencies for voice and data and so as to promote the widespread implementation of DSL by reducing the costs associated with such an implementation.